British patent application No. 8234318 discloses a so-called compound helicopter in which two bypass type gas turbine engines are installed in a helicopter provided with augmentor wings. Bypass air is directed from the engine via ducting to each augmentor wing which is adapted to augment the thrust and lift of the helicopter rotor blades. Such augmentor wings are more particularly described in U.S. Pat. No. 3,332,644 to Donald C Whittley.
The gas turbine engines of the compound helicopter mentioned above are of the free power turbine configuration. A low pressure power turbine is coupled to the helicopter rotors while a high pressure turbine drives the engine compressor. A variable area final nozzle is provided to vary the pressure drop across the power turbine and in order to control the power output to the rotor blades. When maximum power is required for hovering or vertical take-off, the final nozzle is set to maximum area to provide a high pressure drop across the power turbine. For maximum forward speed the final nozzle area is set to a minimum to increase the momentum of the exhaust gases to give maximum forward thrust.